There is great demand for materials resistant to sea water, brackish water or other solutions that contain chlorides or other halide ions. Examples are equipment for surface vessels and submarines and for various chemical handling and power plant applications.
At the present time the two most important known elements that confer chloride resistance to iron-base or nickel-base alloys are molybdenum and chromium.
For maritime applicaton, an alloy has been considered generally satisfactory if it resists corrosion by seawater at ambient temperatures. Recently, however, the extensive use of seawater or brackish water as a cooling medium in heat exchangers has increased, with the result that there is great demand for materials that resist damage by both seawater and the process fluids that are being cooled. In some cases, the process fluid is highly corrosive to many materials, even to some that are able to resist seawater attack. Much progress has been made in developing materials with the required corrosion resistance and other properties. However, such materials have tended to be quite expensive, high in critical or strategic element content, and difficult to prepare and fabricate. Thus, there is great interest in the development of lower cost alloys that are more effective or more efficient than those presently in service in resisting attack by seawater and process fluids.
There is also the desirability in some applications that such alloys be substantially nonmagnetic. One such application is for naval mine-sweepers which must avoid destruction by magnetic mines. Nonmagnetic alloys are also advantageous materials of construction for submarines, since they allow the vessel to elude the magnetic anomaly detector systems that are employed to locate submerged submarines. These systems sense changes in the earth's magnetic field caused by metallic masses as large as steel submarines.
The element titanium and its principal alloys are nonmagnetic, are totally immune to ordinary seawater attack, and have been employed in the hulls of a few submarines and in the heat exchanger tubes of a few seawater-cooled power plants. However, titanium is relatively scarce and expensive, quite difficult to fabricate, and very susceptible to contamination and embrittlement if processed by conventional methods. Hence, Ti weldments tend to crack and leak, and Ti cannot be melted and cast into shapes except under the most rigorous conditions in vacuum or inert gas atmospheres. Also, use of titanium tubing in retrofitting existing heat exchangers may lead to excessive vibration failures unless dampeners are used or support sheets are repositioned.
Thus, there is continued interest in air meltable, castable, weldable, fabricable alloys to resist attack by sea water, and for many applications that remain essentially non-magnetic.
In spite of their excellent overall corrosion resistance, the usual commercial stainless steels are subject to localized corrosion in stagnant seawater. Stagnant conditions arise when the flow rate over the metallic surfaces is less than about 1.2 to 1.6 meters per second (3.9 to 5.2 feet per second), when marine organisms are attached to the surfaces, or where crevices exist. Such conditions are very difficult to avoid completely in actual practice. Thus, although general corrosion of stainless steel components tends to be very low in seawater, very serious damage leading to early failure often occurs because of localized corrosion.
Pitting attack and penetration or perforation of stainless steels tend to take place on broad surfaces with low fluid flow rates, while some form of crevice corrosion takes place where there are imperfect contacts with mud, fouling substances, wood, paint, or other bodies, or even where there are reentrant angles or corners.
A major obstacle to the use of austenitic stainless steels for service in strong chloride environments has been the possibility of chloride stress corrosion cracking. Under conditions of even moderate stress and temperature, type 304 (ordinary 18% Cr 8% Ni) stainless steel will crack at very low chloride levels. Stress corrosion cracking has not really been well understood in the past, but it is now known that improved and highly modified stainless steels of higher molybdenum contents above 3.5% have a degree of resistance to chloride stress corrosion cracking that is more than adequate for most high chloride service.
Ferritic iron-chromium-molybdenum stainless steels have been developed for chloride service, but they must be produced with extrememly low carbon and nitrogen contents, and are hence not available as cast shapes by ordinary production methods. They are also magnetic and readily attacked by many common chemicals.
A number of austenitic, nonmagnetic, nickel-base alloys have also been developed for the same types of service. These have contained up to 31% Cr, up to 28% Mo, and from about 45% to about 65% Ni. They often contain 1 to 4% W and up to 4% Cb (Nb) plus Ta, iron being limited to levels around 2 to 6% maximum. The combined proportion of Mo, Cr and W in these alloys varies between about 32 and 42% total. Such high levels of the latter elements require very high nickel and low iron contents in order to maintain a single-phase austenitic (face-centered-cubic) crystal structure. While these nickel-base alloys are castable, weldable, and fairly fabricable, they must be produced from high-purity, extremely expensive and relatively scarce melting stock.
My copending application, Ser. No. 947,427, filed Dec. 29, 1986 describes an iron-base alloy of approximately 18% Cr, 7.5% Mo and certain other elements. This alloy is suitable for use in seawater and is also resistant to many corrosive process fluids. As a consequence, it is useful not only in numerous marine applications, but also in process heat exchangers in which other corrosive fluids are cooled with seawater. However, while the alloy of my copending application has been shown to resist various chemical solutions in the presence of or including chlorides or sea water, it remains desirable to provide the same chloride-resistance with lower Mo content. Commercial alloys for chloride service may contain from 17% to 21% Cr and usually over 5 or 6% Mo. All of these resist various corrosive substances in addition to sea water. However, other corrosive substances readily attack such alloys. For example, process streams generated in the production and handling of phosphoric acid are quite corrosive to equipment constructed of such alloys. Another case is equipment for handling many concentrations of pure or impure sulfuric acid. Another very important case is alloys used in off-shore wells for the recovery of "sour" gas and oil as found in the Mid-East oil fields and elsewhere. "Sour" means that the fluids are high in hydrogen sulfide and mercaptan contents. In addition to sulfur compounds, deep sour wells also encounter carbon dioxide, salts such as brine, and temperatures up to 300.degree. to 500.degree. F. (150.degree. to 260.degree. C.). These are all examples of severe service conditions in which chloride resistance of my prior alloy or of the usual commercial chloride-resistant alloys may not be adequate.